Saw blade



June 17, 1947 H, A, -r 2,422,561

SAW BLADE Original Filed July 26, 1940 2 Sheets-Sheet l INVENTOR. lib/71?? 4. 59w)? June 17, 1947.

A. PAVlTT SAW BLADE Original Filed July 26, 1940 2 Sheets-Sheet 2 7 EMPE 19/6' 7IVPEF470F INVEVTOR. RR YAPA v/ 77' Patented g g A A UNITED STATES PATENT. OFFICE 2,422,501 saw Harry a. Pavitt, Fltchhurg, Mass, assignor to Simonds Saw and Steel 00., Fitchburg, Mass., a corporation of Massachusetts Original application 1111! 28, 1940, Serial No. 347,668. Divided and this application May 8, 1943, Serial No. 486,476

8 Claims. (Cl. 143-133) 2 This invention relates to saw blades and aims growth is prevented and the grain size in the in particular to provide an improved hack saw structure 01' the steel is kept unusually small. In blade composed 01' high speed tool steel and havthis way, extreme hardness is obtained, and yet ing hardened teeth. the steel does not suiler a loss of toughness or In the course of heat-treating such blades it ii. become brittle. has been the practice to heat them above the Cfltf' While I do not wish to be limited to any parical temperature and hold them at such elevated tlcular theory, I believe that the ability oi the temperature for a substantial period of time. carbides or carbon components to dissolve in By heating the steel to above the critical teme the. austenitic matrix practically as soon as the perature a solid solution or iron and; carbon. .10 steel reaches the critical temperature is due to known as ti-Imm s formed. d yh si the effector the current in heating the carbides the steel at such elevated temperature the segor carbon components of the steel more rapregated carbides or carbon components of the, idly than the metallic matrix is heated. That Steel Braduauy dissolve in the flustemuc matrixisto y. the carbides or carbon components have This n ed h g s n necessary 111 agreater resistance to the current, and to the Order to Obtain requisite hardness. will rapid reversal or the current when induced by sulted in the steel 105mg mughngss 3 becomf 1 ahlgh-lrequency alternating current, than has i the metallic matrix. Thus, the carbides or car- Thus, in treating such blades according to the hon com nents of the steel reach the tem prior art it has been necessary tolcompmmise ature req lfired for developing in them suflic ient 7 between a heat treatment which produces maximobmty for dissolving m or being absorbed in mum hardness but with an undesired loss of toughness, and a heat treatment which produces. g z tg fs g lg ii t g: $2

igg figgg g g ig zfi g' ggfi gfigg g: current through the steel, the particles at the m I center 01- the section which is heated are subor the performance of cutflngtools depends 'Jected to'the same heating as are the particles not only on the hardness but also on the toughat the outside, as distinguished from the type oi ness of the steel. If the steel is toosoft, then the heat 8 I l [c] the heat is applied to the tool wears excessively; it the steel is too brittle, side of the steel and m which the heat is com th 't hi gllhigiee 101 c) v (1:12 anti; gireallzsen the teeth of a toward the center of the steel relatively ow saw are heated so that the carbides or carbon Other features, objects and advantages of the components of the steel are in condition to dissolve in the austenitic matrix as soon as the steel invention Wm be pointed mm or become apparent in connection with certain specific examples attains the critical temperature, or a slightly as higher temperature, a substantially complete sowhich are set by way mustra'tmn- The lution of the carbides or carbon components of invention has been developed in connectmn with the steel can be obtained without the need for hack saw bladesflnd will be 1uugtmtell by refer holding the steel at or above the critical ence to such blades merely by way 01' example. perature for any substantial length or time. 40 In describing the specific mp s ef n More specifically, I have discovered that h will be had to the accompanying drawings wherethe teeth of the saw are rapidly heated by a highin:

frequency induced current, passed through the s- 1 is aside elevation f one ime of appsteeth, it is unnecessary to hold the steel at or raw for heating the cutting edge of a saw above the critical temperature ior any substanblade;

tial length of time in order to'p 'oduce a steel Fig. 2 is an end view of the heatin 011 and substantially free from undissolved carbides. a blade wn in ls. 1;

I hav foun that wh high speed t 1 Fig. 3 is a diagrammatic side view representing is heated in this manner, and cooled without be- 8 saw n which y the t e h h ve been harding held at or above the critical temperaturefcr fined; any substanital length or time such as would 4 is a d a ra mat side view r pr sentproduce undesirable grain growth, the resulting ing a saw in which the teeth, the Bullets and a steel exhibits,-under microscopic examination,'a portion of the blade have been heat ,treated; substantial or complete absence of undissolved Fig. 5 is a graph showing the eilect of tempercarbides, while at the same time undesired grain 66 ing on high speed tool steels hardened, respecblade with the teeth I 3 tively, in accordance with the invention and in accordance with the prior art; v

Fig. 6 is a photomicrograph of a section oi steel hardened in a conventional prior art man-v low copper tubingand havingspaces 2 between the convolutions or the coil. An alternating current may be applied to the coil in any convenient mannerjas will be understood by those skilled in the art, and a cooling liquid is circulated. through the coil to prevent overheating. Means are provided for feeding a saw blade 4 so that its cutting edge I passes between adjacent convolutions 5, e or the induction coil and through field 8 of the coil. Such means may include, for example, a table provided with feed rolls ll rotated in opposite directions nism (not shown). The back 9 of the blade t rests on the table and is fed forward by the feed rolls l I. The teeth I of the blade pass between the convolutions of the induction coil at a height where the-induced current is utilized to best advantage. The extent to which the blade lies between the convolutions of the induction coil will depend upon the zone of the blade which is to be hardened. In some cases it is desired to harden only the teeth I of the blades, and not the gullets, while in other cases it may be desirable toharden or partially harden the gullets. For this purpose, the height of the table may be varied. A blade with the teeth only hardened is shown in Fig. 3. In this blade there is a rather sharp line of demarcation l3 between the hardened and unhardened portions of the blade. A hardened'and the gullets partially hardened is shown in Fig. 4. In this case, there is a rather wide zone I! or medium hardness between the teeth and the blade proper. Such hardening can be readily controlled by subjecting the blade, or the zone of the blade which is to be hardened, to th'area in the induction coil where there is more or less induced current.

I have found that excellent results are obtained by treating high speed tool steels in accordance with the invention. One such high speed tool by any convenient mechaof the same analysis heated to 2375" F. by induc tion heating and then cooled without being held for any substantial time at said elevated temperature. It will be noted that whereas both of these steels show an ASTM grain size of about "1 /2 (15 grains intercepted in 5 inches at a magnification of 1000 diameters), the steel made according to prior practice shows a large amount of undissolved carbides, whereas the steel treated according to the present invention is substantially free from undissolved carbides. That is to say, in order to obtain a small grain size according to prior practice it has'been necessary to sacrifice carbide solubility, with the result that such tools have possessed relatively poor wear resistance. This disadvantage is obviated by the present invention.

As any indication of specific satisfactory conditions, I have found that when a blade of the above analysis, .065 inch thick, is fed through the coil at a rate of about A; of an inch per second, with a power input of 6 kilowatts and a frequency of about 250,000 cycles per second, each inch of length of the blade will be heated to 2375 F. in about 2 seconds, or at an average rate in the order of 950 per second. It will be understood, however, that the rate of feed and power input may be adjusted depending upon the thickness of the blade. It will also be understood that over- I heating is to be avoided, but that the rate of feed should not be excessive, so that time is allowed for heating the steel throughout, to critical temperature.

After the blades have been withdrawn from v the induction coil, they are promptly cooled below the critical temperature to prevent undesired grain growth. High speed tool steels may be aircooled or quenched in water or oil.

In order to observe the exact temperature of the steel during the heating an optical pyrome ter may be -employed. While I prefer to use a high-frequency current, in the order of magnitude mentioned and possibly up to 500,000 cycles per second, no precise frequency is essential to obtaining good results. If desired, the apparatus may be enclosed within an inert atmosphere or liquid, although this precaution appears to be unnecessary. The temperature to which the steel is heated depends upon the analysis of the particular steel. In the case of high speed tool steels I prefer to heat these steels to between 2375 F.

' and 2400 F., and in some cases as high as 2425 F.

steel has the following analysis:

' Example 1 Usual Range Per cent per cent 75 0 to 1.00 18.00 16 to 19 4. 00 3 to 5 Vanadium" 1.50 0 5 to 3.0 Iron containing the usual decxidizers such as silicon and manganese and the usual impurities Balance 100 By rapidly heating a hack saw blade made of high speed steel to about 2375 F. in the apparatus described, and then cooling it without holding it at this temperature for any substantial length of time, a steel can be produced which is free from undissolved carbides and which shows no appreciable increase in grain size. This is shown by a comparison between the two photomicrographs, Figs. 6 and '7. Fig. 6 shows a steel heated according to the prevailing practice, i. e., soaked at the critical temperature for a number of minutes, and then cooled. Fig. '1 shows a steel It will be noted that this temperature is slightly higher than that used in the prior art (2300 F.- 2350 F.). This higher temperature is permissible since the steel is not held at such temperature for any substantial length of time.

A special advantage of the invention in the hardening of high speed tool steels lies in their ability to reach a secondary peak hardness during the usual tempering operation which follows the treatment described. This advantage is illustrated in Fig. 5, in which curve A shows the hardness obtainable by tempering such steel which has been heat treated according to the invention. At approximately 1050 F. this steel attains a secondary peak hardness as great as it had before the tempering. Curve B shows the effect of the tempering of steel which has been heat treated according to prior practice, and indicates that the previous hardness is not restored at any temperature used in the tempering operation.

The invention may be appliedto various high speed tool steels containing substantial proportions of one or more of the elements tungsten,

molybdenum, chromium, or vanadium, with or without the addition of cobalt. Accordingly, when the term "high speed tool steel is used herein it intends a steel containing at least one of these elements in amount in the order of 3 to 25% of the steel.v Merely by way of example, there is set forth the following analysis of another representative high speed tool steel:

The rapid heating of high speed tool steel in the manner described, by mean of high-frequency alternating current, produces a, remarkable result in that the carbides or carbon components of the tool steel are rendered, due to inincreased wear resistance or cutting life together with suiiicient toughness so that even less breakage occurs than in the use of previously known blades having considerably less cutting ability. As already indicated, however, the invention is not limited to hack saw blades.

This application is a division of my eo-pending application, Serial No. 347,666 filed July 26, 1940, and issued Aug. 10, 1943 as Pat. No. 2,326,674.

The terms and expressions which have been employed are used a terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described, or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. What I claim is:

1. A steel saw blade having a hardened zone including the teeth of the saw blade, substantially all portions of the steel within said zone both on the surface and in the interior being of uniform character under magnification X1000, being free from grain coarsening of the type which occurs when the steel is held above the critical tempera- .ture for a substantial length of time, and being creased atomicmobility, soluble and difl'usible in i the steel matrix practically as soon as the steel reaches the critical temperature and before substantial grain growth occurs. Hence, the desired hardness can be produced'without substantial increase in the grain size of the steel, and the grain size, in a steel of given hardness, can be readily controlled by limiting the time the steel is held at high temperature. This can be readily accomplished by designing the apparatu so that, if desired, the tool can be held at or above critical temperature for a predetermined time after it has reached this temperature. Thus, in the case-of steelsof certain analysis it may be desirable to hold these steels momentarily at elevated temperature in order to develop maximum hardness with only a small grain growth. The heating can be readily varied for steels of different analyses by adjusting the rate of feed or by adjusting the power in the induction coil. By thus controlling the heating in accordance with the invention it is possible to produce saws, the teeth of which are substantially free from undissolved carbides, without substantial increase in grain size due to hardening. Still another advantage incident to the use of high-frequency induced current i the ability to zoneharden a section of the saw, thus reducing shattering of the teeth. Also the internal and external portions of the article are heated uniformly. That is to say, every small portion of the steel in a given zone is subjected to the same heating, while the carbide atoms or molecules within each such portion are heated more rapidly than the ad- Jacent matrix, by remn of the relatively high resistance of the carbides. The internal heating of the steel by induced current, as distinguished from external heating, thus results in applying more heat to the carbon components than to the metallic components, in contrast to external heating in which the metallic components conduct heat better than the carbon components.

Hack saw blades have been produced, in accordance with the invention, having markedly substantially free from undissolved carbides, and

the balance of the blade having different characteristics from said zone containing the teeth.

2. A saw blade formed of high speed tool steel" and having teeth which are uniformly hard throughout all portions thereof, free from grain.

coarsening such as occurs when such steel is held above the critical temperature for a substantial length of time, and substantially free from undissolved carbides; and a less hard portion of the blade Joined to the teeth.

3. A hack saw blade composed of steel consisting of approximately .65 to 1.00% carbon, approximately 3 to 5% chromium, approximately vanadium, one of the group consisting of approximately 16 to 19% proximately 7 to 9% molybdenum, the balance being iron, as described, said blade having a hardened zone of substantial width and-substantial thickness. said zone including the teeth of the saw, substantially all portions of which, both on the surface and in the interior, being of uniform charactervunder magnification X1000, being free from grain coarsening of the type which occurs when steel is held above the critical temperature for a substantial length of time, and being substantially free from undissolved carbides, and said blade having a less hard zone adjacent said hardened zone containing the teeth.

- HARRY A. PAVI'II.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES'PATENTS Number OTHER REFERENCES Page 158 of Canadian Chemistry and Metallurgy, May 1938. I

tungsten and ap- 

